Flow nebulizers are widely used for sample introduction in inductively coupled plasma (ICP) mass spectrometry (MS) and optical emission spectrometry (OES). Nebulizers generate a fine aerosol, or mist, of liquid droplets that are subsequently vaporized. The aerosol can be generated by passing a high-velocity sheath gas over the orifice of a liquid filled capillary. Due to the Venturi effect, liquid is pulled from the capillary orifice and the surface tension is disrupted to generate a fine liquid aerosol.
In the current art, one existing type of nebulizer is a poly-tetrafluoroethylene (PTFE) nebulizer manufactured by Elemental Scientific, Omaha, Nebr. USA. It can generate an aerosol at 30 μL min−1 liquid flow rate and 1000 mL min−1 gas flow rate. The gas flow into the nebulizer is controlled with a rotameter, or mass flow controller, while liquid flow is commonly generated with a syringe pump, a peristaltic pump, a dual piston high pressure liquid chromatography pump (HPLC), or through self-aspiration. Each nebulizer is calibrated to a specific gas flow for self-aspiration, eliminating the need for an external pump. However, self-aspiration can be affected by tip fouling, viscosity, and environmental changes. Thus, a syringe pump is typically used to maintain a constant liquid flow in the milliliters per minute range.
However, existing nebulizers utilizing syringe pumps can be limited because it is relatively difficult to generate a pulse-free liquid flow with a syringe pump at flow rates less than 100 μL min−1. At relatively low flow rates, oscillations appear due to the stepper motor of the syringe pump. These flow rate oscillations can adversely affect the quality of trace concentration (sub-parts-per-billion) vapors of compounds, but are particularly problematic for low volatility compounds such as explosives. The oscillations in liquid flow yield oscillations in compound vapor concentration that can be readily detected by modern instrumentation and next generation sensors. A stable, pulse or oscillation-free trace vapor stream is necessary for calibration and verification of analytical instrumentation.
Accordingly, there remains a need in the art for a system that can generate a constant, stable vapor stream of liquid solutions with extremely low vapor pressures. That is, the system should be able to provide a continuous, pulse-free flow of liquid solution over several hours of operation.